Generally a lubricant is used at a sliding portion and a rotational portion of a bearing and the like used for cars and machines represented by industrial machines.
Normally grease is filled inside a rolling bearing to lubricate frictional surfaces between rolling elements thereof and inner and outer rings thereof as well as a cage thereof. To prevent the grease filled inside the rolling bearing from flowing to the outside and dust and water from penetrating thereinto, the rolling bearing is provided with a sealing apparatus. But it is difficult to completely enclose the grease in the rolling bearing even in the rolling bearing provided with the sealing apparatus. When the rolling bearing is used for a long time, there is a possibility that the grease gradually flows out and gradually deteriorates due to water which has penetrated thereinto from outside.
The lubricant is classified into a liquid lubricant and a solid lubricant. To solve the problem of the defective sealing of the grease and the problem of the method of preventing the deterioration thereof, grease having a configuration-holding performance brought about by thickening lubricating oil and a solid lubricant retaining the liquid lubricant and thus capable of preventing the liquid lubricant from scattering and dripping are known.
For example, the solid lubricant having the property formed by mixing ultra-high-molecular-weight polyolefin or urethane resin and hardener for it with lubricating oil or grease so that the liquid lubricating component gradually exudes with the liquid lubricating component kept retained between molecules of the resin is known (see patent documents 1 through 3).
The self-lubricating polyurethane elastomer formed by allowing a reaction between polyol and diisocyanate which are polyurethane materials in the lubricating component in the presence of the lubricant is also known (see patent document 4).
When these solid lubricants are solidified by enclosing them in the bearing, the lubricating oil gradually exudes therefrom. These solid lubricants are intended to eliminate the need for maintenance for replenishing the lubricating oil and contribute to the prolongation of the life of the bearing in a strict environment where a large amount of water is present and an environment where a high inertial force acts.
But the rolling bearing having the conventional solid lubricant filled therein has a disadvantage that the rolling bearing cannot be used because it has a short life, is liable to have seizing in a high-speed rotation, and fuses the matrix consisting of the resin component owing to the generation of heat. A full-pack specification has a problem that in the process of cooling the above-described solid lubricant solidified in the bearing, the lubricant itself surrounds tightly the rolling elements because the solid lubricant contracts and thus the rotational torque is liable to become high and heat is liable to be generated.
In recent years, technical improvement for allowing cars to have a high performance, be compact, and be light weight has progressed. The process of the present invention for producing a universal joint includes the steps of enclosing the lubricant for initial lubrication inside the universal joint; filling a mixture essentially containing a resin which foams, hardens, and becomes porous and a lubricating component inside the universal joint in which the lubricant for initial lubrication is enclosed and foaming and hardening the mixture to form the foam lubricant. Therefore there is a growing demand for research and development of new lubricants and lubricating mechanisms.
But when the solid lubricant is used at portions such as the driving portion of the constant velocity universal joint where an external force such as compression, flexure, and the like is repeatedly applied at a high frequency, a very large force is necessary to deform the solid lubricant in conformity to the compression, flexure, and the like or a very high stress is applied to the solid lubricant, therefore a portion retaining the solid lubricant requires a high mechanical strength.
But the strength of the solid lubricant and the filling rate thereof make compensation for each other. Therefore it is difficult to retain the solid lubricant at a high filling rate. Thus there is a possibility of preventing the constant velocity universal joint from having a long life.
Therefore there is a demand for the development of a solid lubricant which can be easily used at the portion to which the external force such as compression, flexure, and the like is repeatedly applied at a high frequency.
As an example of the solid lubricant for complying with such a demand, the known solid lubricant is used by impregnating the lubricating oil into the flexible resin foamed to form communicating pores and filling the foam lubricant retaining the lubricating oil in the communicating pores in the bearing and the constant velocity universal joint (see patent document 5). The foam lubricant is compressed in conformity to the boot which is deformed by the flexure of the universal joint. The liquid lubricant which has exuded from the foam lubricant is supplied to a necessary portion to allow preferable lubrication.
But the solid lubricant disclosed in the patent documents 1 through 4 have a high force of retaining the lubricating component such as the lubricating oil and grease, but lack in a flexible deformability. The method of preparing the foam lubricant disclosed in the patent document 5 in the communicating pores is of a later-impregnation type, that is, impregnating the foamed resin with the lubricating oil. When the above-described solid lubricant is used, it has a low lubricating oil-retaining force because the lubricating oil is not contained in the solid component. Thus when the solid lubricant is used for the bearing or the universal joint in a high-speed operation condition, there is a possibility that the lubricating oil exudes rapidly from the foamed resin and runs dry. The above-described foam lubricant can be used for lubrication in a short period of time and in a closed space. But when the foam lubricant is used at a portion which requires long-time lubrication or in an open space, the lubricating oil is insufficiently supplied. When the foam lubricant has a low lubricating oil-retaining force, an excess of the lubricating oil is repeatedly discharged from the pores and absorbed thereto, thus always flowing in the space. In such a case, in dependence on the chemical property of the lubricant and an additive contained therein, there is a possibility that they attack and deteriorate the boot material of the universal joint. Thus the foam lubricant is defective in that the material of the boot is limited to a specific kind or the materials of the lubricant and/or the additive are limited to specific kinds. In addition, there is a later impregnation-caused increase in the number of production steps, an increase in the production period of time, and consequently an increase in the production cost.
For the above-described reason, there is a demand for the development of the lubricant, for use in the bearing and the constant velocity universal joint, having a high force of retaining the lubricating component such as the lubricating oil and the grease and a high extent of deformation. It is particularly necessary for a solid component to contain the lubricating component to enhance the force of retaining the lubricating component.
The foam lubricant in which the lubricating component is contained in the solid component to enhance the force of retaining the lubricating component can be supplied in a necessary amount and to a necessary portion better than grease lubrication industrially widely used. Therefore the art of allowing the solid component to contain the lubricating component therein has advantages of decreasing the cost owing to a decrease in the use amount of grease, decreasing a load to be applied to the material for the boot, decreasing the weight of the constant velocity universal joint, and making the constant velocity universal joint compact. Thus it can be said that this art is high in the degree of importance for society from the standpoint of not only an economic side but also decreasing a load to be applied to environment and in addition increasing the degree of freedom in design.
Even in the bearing and the constant velocity universal joint in which the foam lubricant having the above-described many advantages is enclosed, in dependence on the manner of using them, there is a case where an initial discharge amount of the lubricant is small due to an external force and a rise of temperature. In consideration of durability, it is desirable that the discharge amount of the lubricating component from the resin component is set to minimum necessary amount. When the discharge speed of the lubricating component is low, a necessary amount of the lubricating component reaches the sliding portion at a low speed. As a result, at an early stage, the lubricating component runs dry and thereby wear and defective lubrication may occur at the rolling portion and the sliding portion. Thereby owing to damage of a sliding surface, portions to be lubricated may have a short life. Therefore there is a demand for the development of a lubricant capable of supplying the lubricating component such as lubricating oil immediately after lubrication starts.
In conventional methods of filling the solid lubricant such as the foam lubricant inside the constant velocity universal joint and the like, in dependence on the flowability thereof, there is a possibility that the solid lubricant is not filled at a necessary portion, but filled in a portion such as a shaft hole of an inner member unnecessary to be lubricated. When the filling amount is very large, there occurs a problem that because owing to the interruption of the solid lubricant, it is difficult to assemble component parts. Particularly in filling the solid lubricant in an internal gap between the inner member and the outer member, it is necessary to fill the solid lubricant to mainly the peripheries of torque transmission members such as a track portion and raceway surfaces thereof. It is unnecessary to fill the solid lubricant in the shaft hole of the inner member. To fill an appropriate amount of the solid lubricant in the portions to be lubricated, it is necessary to adjust a pressure to be applied to the solid lubricant at a filling time and an extrusion amount. The work of securely accomplishing these operations is troublesome and not easy.    Patent document 1: Japanese Patent Application Laid-Open No. 6-41569    Patent document 2: Japanese Patent Application Laid-Open No. 6-172770    Patent document 3: Japanese Patent Application Laid-Open No. 2000-319681    Patent document 4: Japanese Patent Application Laid-Open No. 11-286601    Patent document 5: Japanese Patent Application Laid-Open No. 9-42297